Because, as stated above (Post No. 9 by Britmet), Bikes and Engines have changed over the years and a lot of Tracks haven't. Some are basically the same as they were back in 1928. The Tracks were OK for Speedway in those days but with the 'Rocket Ships' Riders are using these days the Sport has become faster (minimally) and more dangerous hence a greater risk of injury.
Certainly I don't believe that the Bikes are that much faster that it justifies all of the cost involved in having them Tuned.